1) Field of the Invention
The Application relates to a process for the preparation of pure melamine according to claim 1.
2) Description of the Related Art
In the high-pressure processes for the preparation of melamine, in general urea melt and optionally gaseous ammonia are reacted without the presence of a catalyst, for example at temperatures between 325 and 450° C., preferably between 350 and 425° C., and pressures between 50 and 250 bar to give liquid melamine and off gas. The reaction off gas mainly comprises ammonia and carbon dioxide, with small amounts of gaseous melamine. In addition to unconverted urea, the liquid crude melamine also contains byproducts, such as, for example, melem, melam and further condensation products of melamine, which are undesired in the end product and therefore have to be separated off. The melamine byproducts are separated from melamine by utilizing the known fact that the byproducts hydrolyze with water, preferably in the presence of alkalis, to give oxoaminotriazine compounds, such as ammeline and ammelide. During the subsequent melamine crystallization, these are kept in solution so that the pure melamine crystallizes out selectively. In these melamine processes, the melamine melt from the high pressure part is worked up in a downstream low pressure part in the presence of water.
According to U.S. Pat. No. 3,132,143, for example, the reaction mixture from the high pressure synthesis reactor, consisting of the melamine melt and the off gas, is fed to a quencher in which the mixture is brought into contact with an aqueous solution saturated with ammonia and carbon dioxide, at from 100 to 200° C. and from 10 to 35 bar for from 10 to 60 min. On contact with the cool quench solution, the melamine is absorbed therein while the major part of the off gas is separated off. For the degradation of the byproducts, the melamine solution is allowed to reside for from 20 to 50 min, the NH3 and CO2 contained is then removed with the aid of steam and, after addition of alkali-containing mother liquor and filtration of insoluble products, the melamine crystallizes out.
A disadvantage of this process is the fact that the melamine off gas is separated from the melamine only in the quencher and is thus obtained at a low pressure level and in a state which is not anhydrous. Since the off gas mainly comprises NH3 and CO2, considerable amounts of CO2 are introduced into the wet part of the plant. CO2 which has already been separated off is even recycled into the melamine process via the CO2-containing quench liquid, with the result that the CO2 content of the melamine solution is increased and hence the pH of the solution is reduced. This is disadvantageous in particular for the byproduct degradation preferably taking place in the alkaline range during the residence in the quencher, since this takes place slowly and incompletely under these conditions. The total CO2 is stripped from the melamine solution only in the steam stripper of the wet part. This is also very energy-consumptive.
WO 00/29393 A1 or WO 03/045927 A1 describes melamine preparation processes in which the melamine melt is separated from the reaction off gas in the high pressure reactor itself. The off gas is obtained in anhydrous form and at high pressure and is recycled into the urea plant. The melamine melt fed for further working-up accordingly already contains a CO2 content reduced by the proportion of off gas. Subsequently, the CO2 dissolved in the melamine melt is removed from the melt, for example by passing through NH3. The melamine melt pretreated in this manner is then fed to the quencher, in which the melamine melt is converted into a melamine suspension or solution by contact with an aqueous, alkali-containing solution. In order to accelerate the byproduct degradation and to keep oxoaminotriazine compounds formed thereby, ammeline and ammelide, in solution, NaOH is added to the melamine solution before it is allowed to reside for byproduct degradation. Dissolved NH3 still present is then stripped out and the melamine finally crystallizes out. A disadvantage of these processes is the fact that considerable amounts of CO2 are still present in the melamine suspension or solution discharged from the quencher. Said amounts arise because of incomplete CO2 removal in the high pressure part and as a result of hydrolytic decomposition of unconverted urea and melamine byproducts in the quencher. The total CO2 present must be neutralized by adding NaOH. Only on further addition of NaOH does the pH increase. A high pH is required for rapid byproduct degradation during the subsequent residence of the melamine solution. This results in very large amounts of NaOH for the desired high melamine purity with low byproduct contents, which amounts are undesired for economic and logistical reasons.
It was accordingly the object to provide a melamine process which has a reduced NaOH consumption in combination with the same energy characteristics of the plant and the same quality of the end product melamine.